Heat treated cold rolled steel sheet and a method of manufacturing thereof

ABSTRACT

A heat treated and cold rolled steel sheet having a composition including of the following elements 0.09%≤Carbon≤0.15%, 1.8%≤Manganese≤2.5%, 0.2%≤Silicon≤0.7%, 0.01%≤Aluminum≤0.1%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, 0%≤Niobium≤0.1%, 0%≤Titanium≤0.1%, 0%≤Chromium≤1%, 0%≤Molybdenum≤1%, 0%≤Vanadium≤0.1%, 0%≤Calcium≤0.005%, 0%≤Boron≤0.01%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite and a cumulative presence of Ferrite and Bainite between 15 and 35%.

The present invention relates to cold rolled steel sheet with highstrength and high formability having tensile strength of 950 MPa or moreand a hole expansion ratio of more than 56% which is suitable for use asa steel sheet for vehicles.

BACKGROUND

Automotive parts are required to satisfy two inconsistent necessities,namely ease of forming and strength. However in recent years a thirdrequirement of improvement in fuel consumption is also bestowed uponautomobiles in view of global environment concerns. Thus, now automotiveparts must be made of material having high formability in order to fitin the criteria of ease of fit in the intricate automobile assembly andat same time have to improve strength for vehicle crashworthiness anddurability while reducing weight of vehicle to improve fuel efficiency.

Therefore, intense Research and development endeavors are undertaken toreduce the amount of material utilized in car by increasing the strengthof material. Conversely, an increase in strength of steel sheetsdecreases formability, and thus development of materials having bothhigh strength and high formability is necessitated.

Earlier research and developments in the field of high strength and highformability steel sheets have resulted in several methods for producinghigh strength and high formability steel sheets, some of which areenumerated herein for conclusive appreciation of the present invention:

JP2012111978 is a patent application having a composition C: 0.05-0.3%,Si: 0.01-3.0%, Mn:0.5-3%, Al: 0.01-0.1%, and the balance Fe andincidental impurities and having a component composition consisting of,ferrite and tempered martensite as a main component of the high-strengthcold rolled steel sheet but such steel is not able to reach more than50% of hole expansion ratio.

EP2971209 is patent that relates to a high strength hot dip galvanisedcomplex phase steel strip having improved formability to be used in theautomotive industry having an mandatory elemental composition C:0.13-0.19%, Mn:1.70-2.50% Si: 0-0.15%, Al: 0.40-1.00%, Cr: 0.05-0.25%,Nb: 0.01-0.05%, P: 0-0.10%, Ca: 0-0.004%, S: 0-0.05%, N: 0-0.007% thebalance being Fe and inevitable impurities, wherein 0.40%<Al+Si<1.05%and Mn+Cr>1.90%, and having a complex phase microstructure, in volumepercent, comprising 8-12% retained austenite, 20-50% bainite, less than10% martensite, the remainder being ferrite but the granted patent isunable to reach the tensile strength beyond 900 MPa.

SUMMARY OF THE INVENTION

The known prior art related to the manufacture of high strength and highformability steel sheets is inflicted by one or the other problems:hence there lies a need for a cold rolled steel sheet having highstrength and high formability and a method of manufacturing the same.

It is an object of the present invention to provide cold-rolled steelsheets that simultaneously have:

-   -   an ultimate tensile strength greater than or equal to 950 MPa        and preferably above 980 MPa, or even above 1000 MPa,    -   a total elongation greater than or equal to 8%.    -   a hole expansion ratio of 56% or more and preferably 57% or        more.

In a preferred embodiment, the steel sheet according to the inventionmay have a yield strength value greater than or above 750 MPa.

Preferably, such steel can also have a good suitability for forming, inparticular for rolling with good weldability and coatability.

Another object of the present invention is also to make available amethod for the manufacturing of these sheets that is compatible withconventional industrial applications while being robust towardsmanufacturing parameters shifts.

DETAILED DESCRIPTION

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

Carbon is present in the steel between 0.09% and 0.15%. Carbon is anelement necessary for increasing the strength of a steel sheet byproducing a low-temperature transformation phase such as martensite. Acontent less than 0.09% would not allow securing adequate amount ofmartensite, thereby decreasing strength as well as ductility. On theother hand, at a carbon content exceeding 0.15%, a weld zone and aheat-affected zone are significantly hardened, and thus the mechanicalproperties of the weld zone are impaired. The preferred limit for Carbonis between 0.1% and 0.14%, more preferably 0.1% and 0.13%.

Manganese content of the steel of the present invention is between 1.8%and 2.5%. Manganese is an element that imparts strength by solidsolution strengthening. An amount of at least about 1.8% by weight ofmanganese has been found in order to provide the strength andhardenability of the steel sheet. Thus, a higher percentage of Manganesesuch as 1.9% to 2.4% is preferred and more preferred limit is between2.0% and 2.3%. But when manganese is more than 2.5%, this producesadverse effects such as slowing down the transformation of austenite tomartensite during the cooling after annealing, leading to a reduction ofstrength. Moreover, a manganese content above 2.5% would also reduce theweldability of the present steel.

Silicon is an essential element for the steel of the present invention,Silicon is present between 0.2% and 0.7%. Silicon is added to the steelof the present invention to impart strength by solid solutionstrengthening. Silicon plays a part in the formation of themicrostructure by preventing the precipitation of carbides and bypromoting the formation of martensite. But whenever the silicon contentis more than 0.7%, surface properties and weldability of steel isdeteriorated, therefore the Silicon content is preferred between 0.3%and 0.7% and more preferably between 0.4% and 0.6%.

Aluminum content of the present invention is between 0.01% and 0.1%.Aluminum is added to de-oxidize the steel of the present invention.Aluminum is an alphageneous element and also retarding the formation ofcarbides. This can increase the formability and ductility of steel. Inorder to obtain such an effect, Aluminum content is required at 0.01% ormore. However, when the Aluminum content exceeds 0.1%, Ac3 pointincreases beyond acceptable, austenite single phase is very difficult toachieve industrially hence hot rolling in complete austenite regioncannot be performed. Therefore, Aluminum content must not be more than0.1%. The preferable limit for the presence of Aluminum is between 0.01%and 0.08% and more preferably 0.01% and 0.05%.

Phosphorus content of the steel of the present invention is limited to0.09%. Phosphorus is an element which hardens in solid solution and alsointerferes with formation of carbides. Therefore a small amount ofphosphorus, of at least 0.002% can be advantageous, but phosphorus hasits adverse effects also, such as a reduction of the spot weldabilityand the hot ductility, particularly due to its tendency to segregationat the grain boundaries or co-segregation with manganese. For thesereasons, its content is preferably limited a maximum of 0.02%.

Sulfur is not an essential element but may be contained as an impurityin steel. The sulfur content is preferably as low as possible, but is0.09% or less and preferably at most 0.01%, from the viewpoint ofmanufacturing cost. Further if higher sulfur is present in steel itcombines to form sulfide especially with Mn and Ti and reduces theirbeneficial impact on the present invention.

Nitrogen is limited to 0.09% in order to avoid ageing of material,nitrogen forms the nitrides which impart strength to the steel of thepresent invention by precipitation strengthening with Vanadium andNiobium but whenever the presence of nitrogen is more than 0.09% it canform high amount of Aluminum Nitrides which are detrimental for thepresent invention hence the preferable upper limit for nitrogen is0.01%.

Niobium is an optional element that can be added to the steel up to0.1%, preferably between 0.001% and 0.1%. It is suitable for formingcarbonitrides to impart strength to the steel according to the inventionby precipitation hardening. Because niobium delays the recrystallizationduring the heating, the microstructure formed at the end of the holdingtemperature and as a consequence after the complete annealing is finer,this leads to the hardening of the product. But, when the niobiumcontent is above 0.1% the amount of carbo-nitrides is not favorable forthe present invention as large amount of carbo-nitrides tend to reducethe ductility of the steel.

Titanium is an optional element which may be added to the steel of thepresent invention up to 0.1%, preferably between 0.001% and 0.1%. Asniobium, it is involved in carbo-nitrides so plays a role in hardening.But it is also involved to form TiN appearing during solidification ofthe cast product. The amount of Ti is so limited to 0.1% to avoid coarseTiN detrimental for hole expansion. In case the titanium content isbelow 0.001% it does not impart any effect on the steel of the presentinvention.

Chromium content of the steel of the present invention is between 0% and1%. Chromium is an optional element that provide strength and hardeningto the steel, but when used above 1% impairs surface finish of thesteel.

Molybdenum is an optional element that constitutes between 0% and 1% ofthe Steel of the present invention; Molybdenum increases thehardenability of the steel of the present invention and influences thetransformation of austenite to Ferrite and Bainite during cooling afterannealing. However, the addition of Molybdenum excessively increases thecost of the addition of ahoy elements, so that for economic reasons itscontent is limited to 1%.

Vanadium is an optional element which may be added to the steel of thepresent invention up to 0.1%, preferably between 0.001% and 0.01%. Asniobium, it is involved in carbo-nitrides so plays a role in hardening.But it is also involved to form VN appearing during solidification ofthe cast product. The amount of V is so limited to 0.1% to avoid coarseVN detrimental for hole expansion. In case the vanadium content is below0.001% it does not impart any effect on the steel of the presentinvention.

Calcium is an optional element which may be added to the steel of thepresent invention up to 0.005%, preferably between 0.001% and 0.005%.Calcium is added to steel of the present invention as an optionalelement especially during the inclusion treatment. Calcium contributestowards the refining of the steel by arresting the detrimental sulphurcontent in globularizing it.

Other elements such as cerium, boron, magnesium or zirconium can beadded individually or in combination in the following proportions:Ce≤0.1%, B≤0.01%, Mg≤0.05% and Zr≤0.05%. Up to the maximum contentlevels indicated, these elements make it possible to refine the grainduring solidification. Present invention does not intend to add Copperand Nickel but these elements may be present as residuals up to 0.1%either severaly or cumulatively.

The remainder of the composition of the steel consists of iron andinevitable impurities resulting from processing.

The microstructure of the steel sheet according to the inventioncomprises in area fractions 65% to 85% of Tempered Martensite, 0% and 5%of residual austenite and cumulative amount of bainite and ferritebetween 15% and 35%. Tempered martensite constitutes the matrix phasefor the steel of the present invention

Tempered Martensite constitutes between 65% and 85% of themicrostructure by area fraction. Tempered martensite is formed from themartensite which forms during the second step of cooling after annealingand particularly below Ms temperature and more particularly betweenMs-50° C. and 20° C. Such martensite is then tempered during the holdingat a tempering temperature Temper between 150° C. and 400° C. Temperedmartensite of the present invention does not contain coarse IronCarbides specifically iron based carbide having a grain size aspectratio of 3.5 or more because these iron carbides inhibits the presentinvention to reach the target hole expansion ratio. The martensite ofthe present invention imparts ductility and strength to such steel.Preferably, the content of martensite is between 65% and 80% and morepreferably between 68% and 78%.

Bainite and Ferrite are cumulatively present in the steel between 15%and 35%. In a preferred embodiment, the range for cumulated amount offerrite and bainite is between 20% and 35% and more preferably between22% and 32%.

Ferrite constituent improves the properties of the steel of the presentinvention, in particular regarding elongation and hole expansion ratioas ferrite is a soft and intrinsically ductile constituent. This ferriteis mainly formed during the first step of cooling after annealing. In apreferred embodiment, ferrite can be present at least in an amount of15%.

Bainite can impart strength to the steel but when present in a largeamount it may adversely impact the hole expansion ratio and elongationof the steel. Bainite forms during the reheating before tempering. In apreferred embodiment, the bainite content is kept between 0% and 10%more preferably below 8% and even more preferably below 5%.

Residual Austenite is an optional phase that can be present between 0%and 5% in the steel, but is preferably not present.

In a preferred embodiment the steel sheet according to the invention maybe obtained by any appropriate method. It is however preferred to usethe process according to the preferred embodiments of the invention,which comprises the following successive steps:

Such process includes providing a semi-finished product of steel with achemical composition according to the invention. The semi-finishedproduct can be casted either into ingots or continuously in form of thinslabs or thin strips, i.e. with a thickness ranging from approximately220 mm for slabs up to several tens of millimeters for thin strip, forexample.

For the purpose of simplification of the present invention, a slab willbe considered as a semi-finished product. A slab having theabove-described chemical composition is manufactured by continuouscasting wherein the slab preferably underwent a direct soft reductionduring casting to ensure the elimination of central segregation andporosity reduction. The slab provided by continuous casting process canbe used directly at a high temperature after the continuous casting ormay be first cooled to room temperature and then reheated for hotrolling.

The temperature of the slab which is subjected to hot rolling is atleast 1000° C., preferably above 1100° C. and must be below 1250° C. Incase the temperature of the slab is lower than 1000° C., excessive loadis imposed on a rolling mill, and further, the temperature of the steelmay decrease to a ferrite transformation temperature during finishingrolling, whereby the steel will be rolled in a state in whichtransformed ferrite contained in the structure. Further, the temperaturemust not be above 1250° C. as there would be a risk of formation ofrough ferrite grains resulting in coarse ferrite grain which decreasesthe capacity of these grains to re-crystallize during hot rolling. Thelarger the initial ferrite grain size, the less easily itre-crystallizes, which means that reheat temperatures above 1250° C.must be avoided because they are industrially expensive and unfavorablein terms of the recrystallization of ferrite.

The temperature of the slab is preferably sufficiently high so that hotrolling can be completed entirely in the austenitic range, the finishinghot rolling temperature remaining above Ac3 and preferably above Ac3+50°C. It is necessary that the final rolling be performed above Ac3,because below this temperature the steel sheet exhibits a significantdrop in rollability. A final rolling temperature is preferably aboveAc3+50° C. to have a structure that is favorable to recrystallizationand rolling.

The sheet obtained in this manner is then cooled down at a cooling rateof at least 30° C./s to the coiling temperature which is below 600° C.Preferably, the cooling rate will be less than or equal to 65° C./s andabove 35° C./s. The coiling temperature is preferably of at least 350°C. to avoid the transformation of austenite into ferrite and pearliteand to contribute in forming an homogenous bainite and martensitemicrostructure.

The coiled hot rolled steel sheet may be cooled down to room temperaturebefore subjecting it to an optional hot band annealing or may be send toan optional hot band annealing directly.

Hot rolled steel sheet may be subjected to an optional pickling toremove the scale formed during the hot rolling, if needed. The hotrolled sheet is then subjected to an optional hot band annealing at atemperature between 400° C. and 750° C., preferably during 1 to 96hours.

Thereafter, pickling of this hot rolled steel sheet may be performed ifnecessary to remove the scale.

The hot rolled steel sheets are then cold rolled with a thicknessreduction between 35 to 90%. The cold rolled steel sheet is thensubjected to annealing to impart the steel of the present invention withtargeted microstructure and mechanical properties.

To anneal the cold rolled steel sheet, the cold rolled steel sheet isheated in a two-step heating process, in step one the cold rolled sheetsis heated to a temperature HT1 between 600° C. and 650° C. at a heatingrate HR1 of at least 10° C./s. Then, in step two, the cold rolled sheetis heated from HT1 to an annealing temperature between Ac3 and Ac3+200°C. at a heating rate HR2 of at least 1° C./s and preferably at least2.0° C./sHR1 is always higher than HR2.

The preferred HR1 is at least 15° C./s and the preferred HT1 temperaturerange is between 600° C. and 630° C. The preferred range for annealingtemperature is between Ac3+10° C. and Ac3+150° C. and more preferablybetween Ac3+20° C. and Ac3+100° C.

Then the cold rolled steel sheet is held at the annealing temperatureduring at least 5 s and not more than 1000 s. The temperature and timeare selected to ensure 100% re-crystallization i.e. to obtain apercentage of 100% austenite at the end of the annealing.

The sheet is then cooled in a three-step cooling process. In step one,the cold rolled sheet is cooled from the annealing temperature to atemperature CT1 between 675° C. and 725° C. at a cooling rate CR1 of 10°C./s or less. Then, in step two, the cold rolled sheet is cooled fromCT1 to CT2 between 450° C. and 550° C. at a cooling rate CR2 of at least30° C./s. Then, in step three, the cold rolled sheet is cooled from CT2to CT3 between Ms-50° C. and 20° C. at a cooling rate CR3 which is atleast 200° C./s.

In a preferred embodiment, the cooling rate CR1 is 5° C./s or less andCT1 is preferably between 685° C. and 720° C. and more preferably 685°C. and 700° C. The preferred range for CR2 is at least 40° C./s and thepreferred range for CT2 is between 450° C. and 525° C. and morepreferably between 460° C. and 510° C. The preferred range for CR3 is atleast 300° C./s and more preferably at least 400° C./s. Preferred limitfor CT3 is between Ms-80° C. and 20° C. and more preferably betweenMs-100° C. and 20° C.

Then the cold rolled steel sheet at a heating rate of at least 10° C./s,or better of at least 20° C./s and to a tempering temperature between300° C. and 380° C. and held at tempering temperature during at least100 s but not more than 1000 s. to obtain tempered martensite,conferring the steel of the present invention with good mechanicalproperties. The preferred tempering temperature range is between 320° C.and 360° C. and more preferably is 330° C. and 350° C.

The cold rolled steel sheet is then cooled to room temperature,preferably at a cooling rate of 200° C./s or less.

An optional skin pass operation with a reduction rate below 1% may beperformed at that stage or an optional tension leveling operation.

The heat treated cold rolled sheet may then be optionally coated byelectrodeposition or vacuum coating or any other suitable process.

An optional post batch annealing, preferably done at 170 to 210° C.during 12 h to 30 h can be done optionally after annealing on uncoatedproduct or after coating on coated product in order to reduce hardnessgradient between phases and ensure degasing for coated products.

EXAMPLES

The following tests and examples presented herein are non-restricting innature and must be considered for purposes of illustration only, andwill display the advantageous features of the present invention andexpound the significance of the parameters chosen by inventors afterextensive experiments and further establish the properties that can beachieved by the steel according to the invention.

Samples of the steel sheets according to the invention and to somecomparative grades were prepared with the compositions gathered in table1 and the processing parameters gathered in table 2. The correspondingmicrostructures of those steel sheets were gathered in table 3 and theproperties in table 4.

Table 1 depicts the steels with the compositions expressed inpercentages by weight.

TABLE 1 composition of the trials Samples C Mn Si Al P S N Nb Ti Cr Mo BA 0.116 2.150 0.482 0.031 0.020 0.011 0.002 0.011 0.017 0.036 0.0020.003 B 0.117 2.120 0.495 0.033 0.02 0.022 0.002 0.011 0.015 0.040 0.0020.004 C 0.114 2.180 0.458 0.032 0.017 0.019 0.002 0.018 0.025 0.0330.002 0.003

Table 2 gathers the annealing process parameters implemented on steelsof Table 1.

Table 2 also shows Ac3 and Martensite transformation Ms temperatures ofthe steel samples. The calculation of Ac3 and Ms is done by usingfollowing formulas:

${{Ac}3({Andrews})} = {910 - {20{3\lbrack C\rbrack}^{\frac{1}{2}}} - {1{5.{2\left\lbrack {Ni} \right\rbrack}}} + {44.7\lbrack{Si}\rbrack} + {10{4\lbrack V\rbrack}} + {31.{5\left\lbrack {Mo} \right\rbrack}} + {1{3.{1\lbrack W\rbrack}}} - {30\left\lbrack {Mn} \right\rbrack} - {1{1\left\lbrack {Cr} \right\rbrack}} - {2{0\left\lbrack {Cu} \right\rbrack}} + {700\lbrack P\rbrack} + {40{0\left\lbrack {Al} \right\rbrack}} + {12{0\left\lbrack {As} \right\rbrack}} + {40{0\lbrack{Ti}\rbrack}}}$Ms(Barbier) = 545 − 601.2 * (1 − EXP(−0.868[C])) − 34.4[Mn] − 13.7[Si] − 9.2[Cr] − 17.3[Ni] − 15.4[Mo] + 10.8[V] + 4.7[Co] − 1.4[Al] − 16.3[Cu] − 361[Nb] − 2.44[Ti] − 3448[B]

Further, the samples were heated to a temperature between 1000° C. and1250° C. and then subjected to hot rolling with finish temperature 890°C. and thereafter were coiled at a temperature below 600° C. The hotrolled coils were then processed as claimed and cold rolled with athickness reduction between 35 to 90%.

Table 2: Process Parameters of the Trials

TABLE 2a ANNEALING and COOLING HR1 HT1 HR2 Annealing Annealing CT1 CR1CT2 CR2 CT3 CR3 Trial Steel (° C./s) (° C.) (° C./s) T (° C.) time (s)(° C.) (° C./s) (° C.) (° C./s) (° C.) (° C./s) I1 A 22 610 2.à 870 112688 2.1 462 64 20 587 I2 B 18 600 2.6 870 130 691 1.8 507 44 20 559 R1 A23 590 3.5 870 102 702 2.1 487 66 20 677 R2 A 19 640 2.1 870 137 679 1.8440 55 20 457 R3 C 18 610 2.4 875 137 660 2.1 460 46 20 478 R4 B 18 6002.6 870 130 680 1.9 505 43 20 556 underlined values: not according tothe invention.

TABLE 2b TEMPERING Heating rate for tempering Tempering Tempering Ms Ac3Trial (° C./s) temp(^(o) C.) time(s) (° C.) (° C.) I1 24 340 197 409 829I2 21 340 228 409 829 R1 27 340 180 409 829 R2 20 340 240 409 829 R3 19320 240 409 829 R4 21 340 228 409 829

Table 3 gathers the results of test conducted in accordance of standardson different microscopes such as Scanning Electron Microscope fordetermining microstructural composition of both the inventive steel andreference trials.

TABLE 3 microstructures of the trials Steel Tempered Ferrite + ResidualSample Martensite Bainite Austenite I1 74.9 25.1 0 I2 72.4 27.6 0 R188.4 11.6 0 R2 63.3 36.7 0 R3 56.8 43.2 0 R4 63.4 36.6 0 underlinedvalues: not according to the invention

Table 4 gathers the mechanical properties of both the inventive steeland reference steel. The tensile strength, yield strength and totalelongation test are conducted in accordance with ISO 6892 standards,whereas to estimate hole expansion, a test called hole expansion isapplied according the standard IS016630:2009. In this test, sample issubjected to punching to form a hole of 10 mm (=Di) and deformed. Afterdeformation, the hole diameter Df was measured and the hole expansionratio (HER) is calculated using the under formula:

HER %=100*(Df−Di)/Di

TABLE 4 mechanical properties of the trials Tensile Yield Total HoleSample Strength (in Strength Elongation Expansion Steels MPa) (in MPa)(in %) Ratio(in %) I1 1030 852 8.2 57 I2 1006 822 9.4 74 R1 1136 994 6.263 R2 945 739 9.9 56 R3 894 678 13.0 54 R4 937 735 11.1 55 underlinedvalues: not according to the invention.

1-19. (canceled).
 20. A heat treated and cold rolled steel sheet havinga composition comprising of the following elements, expressed inpercentage by weight: 0.09%≤Carbon≤0.15% 1.8%≤Manganese≤2.5%0.2%≤Silicon≤0.7% 0.01%≤Aluminum≤0.1% 0%≤Phosphorus≤0.09%0%≤Sulfur≤0.09%. 0%≤Nitrogen≤0.09% and optionally one or more of thefollowing elements 0%≤Niobium≤0.1% 0%≤Titanium≤0.1% 0%≤Chromium≤1%0%≤Molybdenum≤1% 0%≤Vanadium≤0.1% 0%≤Calcium≤0.005% 0%≤Boron≤0.01%0%≤Cerium≤0.1% 0%≤Magnesium≤0.05% 0%≤Zirconium≤0.05% a remainder of thecomposition being composed of iron and unavoidable impurities caused byprocessing, a microstructure of the steel sheet comprising in areafraction, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite anda cumulative amount of Ferrite and Bainite between 15 and 35%.
 21. Theheat treated and cold rolled steel sheet as recited in claim 20 whereinthe composition includes 0.3% to 0.7% of Silicon.
 22. The heat treatedand cold rolled steel sheet as recited in claim 20 wherein thecomposition includes 0.01% to 0.08% of Aluminum.
 23. The heat treatedand cold rolled steel sheet as recited in claim 20 wherein thecomposition includes 1.9% to 2.4% of Manganese.
 24. The heat treated andcold rolled steel sheet as recited in claim 20 wherein the compositionincludes 0.1% to 0.13% of Carbon.
 25. The heat treated and cold rolledsteel sheet as recited in claim 20 wherein the cumulated amount ofSilicon and Aluminum is between 0.3% and 0.8%.
 26. The heat treated andcold rolled steel sheet as recited in claim 20 wherein the cumulatedamount of Ferrite and Bainite is between 22% and 35% wherein thepercentage of Ferrite is at least 15% of total area fraction of steel.27. The heat treated and cold rolled steel sheet as recited in claim 20wherein the carbon content of residual austenite is between 0.7% and0.9%.
 28. The heat treated and cold rolled steel sheet as recited inclaim 20 wherein the tempered martensite is between 65% and 80%.
 29. Theheat treated and cold rolled steel sheet as recited in claim 20 whereinthe Bainite is between 0% and 10%.
 30. The heat treated and cold rolledsteel sheet as recited in claim 20 wherein the steel sheet has anultimate tensile strength of 950 MPa or more, and a total elongation of8% or more.
 31. The heat treated and cold rolled steel sheet as recitedin claim 30 wherein the steel sheet has an ultimate tensile strength of1000 MPa or more and a hole expansion ratio of greater of at least 55%.32. A method of production of a heat treated and cold rolled steel sheetcomprising the following successive steps: providing a semi-finishedproduct having a composition comprising of the following elements,expressed in percentage by weight: 0.09%≤Carbon≤0.15%1.8%≤Manganese≤2.5% 0.2%≤Silicon≤0.7% 0.01%≤Aluminum≤0.1%0%≤Phosphorus≤0.09% 0%≤Sulfur≤0.09%. 0% ≤Nitrogen≤0.09% and optionallyone or more of the following elements 0%≤Niobium≤0.1% 0%≤Titanium≤0.1%0%≤Chromium≤1% 0%≤Molybdenum≤1% 0%≤Vanadium≤0.1% 0%≤Calcium≤0.005%0%≤Boron≤0.01% 0%≤Cerium≤0.1% 0% ≤Magnesium ≤0.05% 0% ≤Zirconium ≤0.05%a remainder of the composition being composed of iron and unavoidableimpurities caused by processing, reheating the semi-finished product toa temperature between 1000° C. and 1250° C.; rolling the semi-finishedproduct in the temperature range between Ac3 and Ac3+100° C. wherein ahot rolling finishing temperature is above Ac3 to obtain a hot rolledsteel; cooling the hot rolled steel at a cooling rate of at least 30°C./s to a coiling temperature below 600° C.; and coiling the hot rolledsteel; cooling the hot rolled steel to room temperature; optionallyperforming scale removal process on said hot rolled steel sheet;optionally annealing is performed on the hot rolled steel sheet between400° C. and 750° C.; optionally performing scale removal process on thehot rolled steel sheet; cold rolling the hot rolled steel sheet with areduction rate between 35 and 90% to obtain a cold rolled steel sheet;annealing the cold rolled steel sheet in two steps heating wherein: thefirst step starts from heating the steel sheet to a temperature HT1between 600° C. and 650° C., with a heating rate HR1 of at least 10°C./s, the second step starts from heating further the steel sheet fromHT1 to an annealing temperature range between Ac3 and Ac3+200° C., witha heating rate HR2 of 1° C./s or more, HR2 being lower than HR1; thenperform annealing at annealing temperature for 5 to 1000 seconds; thencooling the cold rolled steel sheet in a three step cooling wherein: thefirst step starts from cooling the steel sheet from the annealingtemperature to a temperature CT1 between 675° C. and 725° C., with acooling rate CR1 of 10° C./s or less, the second step starts fromcooling further the steel sheet from CT1 to CT2 between 450° C. and 550°C., with a cooling rate CR2 of 30° C./s or more, the third step startsfrom cooling further the steel sheet from CT2 to CT3 between Ms-50° C.and 20° C., with a cooling rate CR2 of 200° C./s or more; then heatingthe cold rolled steel sheet at a heating rate of at least 10° C./s to atempering temperature between 300° C. and 380° C. and tempered during100 s to 1000 seconds; and then cooling the cold rolled steel sheet toroom temperature range to obtain a heat treated and cold rolled steelsheet.
 33. The method as recited in claim 33 wherein the coilingtemperature is between 350° C. and 600° C.
 34. The method as recited inclaim 33 wherein the HT1 temperature is between 600° C. and 630° C. witha HR1 heating rate of at least 15° C./s.
 35. The method as recited inclaim 33 wherein annealing soaking temperature is between Ac3+10° C. andAc3+150° C.
 36. The method as recited in claim 33 wherein the temperingtemperature range is between 320° C. and 360° C.
 37. A method for themanufacture of a structural or safety part of a vehicle comprisingperforming the method as recited in claim
 33. 38. A method for themanufacture of a structural or safety part of a vehicle comprisingemploying the steel sheet as recited in claim
 20. 39. A vehiclecomprising the structural or safety part obtained by performing themethod as recited in claim
 37. 40. A vehicle comprising the structuralor safety part obtained by performing the method as recited in claim 38.